ex12 - https://medium.com/@lu4nm3/monad-transformers-vs-mtl-d3a1167e2649

ex12_mvm.scala

def program[F[_]](R: MonadReader[F, Env],
                  W: MonadWriter[F, Vector[SystemState]],
                  S: MonadState[F, SystemState]): F[(Vector[SystemState], SystemState)] = {
  implicit val M: Monad[F] = R.monad

  for {
    status <- R.reader(getServerStatus[F]())

    state <- S.get

    env <- R.ask
    
    updatedState = SystemState(state.value + (env -> status))
    _ <- S.set(updatedState)

    _ <- W.tell(Vector(updatedState))

    _ <- status match {
      case Degraded | Unavailable => R.reader(alertAdmin[F](status))
      case _ => M.unit
    }

    logs <- W.logs
    finalState <- S.get
  } yield (logs, finalState)
}